Spin-polarized tunneling is investigated in magnetic tunnel junctions containing an ultrathin interfacial layer of Co:TiO2 magnetic semiconductor. The Co:TiO2 layers (0 to 1 nm thick) are inserted at the SrTiO3/Co interface in La0.67Sr0.33MnO3/SrTiO3/Co tunnel junctions. For all junctions we find a negative tunnel magnetoresistance, which decreases upon the insertion of Co:TiO2, while the junction resistance increases strongly. This suggests that the ultrathin Co:TiO2 is a paramagnetic insulator that acts as an additional tunnel barrier, in contrast to thick (180 nm) layers grown under comparable conditions, which exhibit metallic impurity band conduction and room-temperature ferromagnetism.